The cylindrical magnetron is used in a large coating machine for coating very large sheets of glass or other materials. One application where these sheets of glass are used is in construction of curtain wall buildings where a single glass sheet can be up to 15 feet wide by about 20 plus feet high. The sheets are run through the coating machine shortly after the glass is manufactured. Thus, these are large-scale machines which must rapidly and evenly coat glass as quickly as it can be manufactured. In addition to the quality of the coating the magnetron deposits upon the glass, dependability and serviceability of the magnetron is of the utmost importance.
This is not an easy task taking into account the constraints of the process that is involved. A cylindrical magnetron sputters material from a rotating target tube onto the glass as it is transported past the target. In order to coat such a large piece of glass the target tube can be up to 15 feet in length and 6 inches in diameter and can weigh 1700 pounds. Another complication is that the sputtering actually erodes the target tube during the sputtering process, so the target tube is constantly changing shape during its serviceable lifetime. The sputtering process can require that an extremely high AC or DC power (800 Amps DC, 150 kW AC) be supplied to the target. This power transfer creates extreme heat in the target tube and the surrounding components, which must be cooled in order assure proper performance and to avoid catastrophic failure of the magnetron. Thus, water is pumped through the center of the rotating target tube at high pressure and flow rate. Efficient and effective sputtering also requires that the process take place in a vacuum or a reduced pressure relative to atmosphere. Thus the rotating target must have a very robust sealing system to prevent the high pressure water from leaking into the vacuum environment.
Rotating such a large target tube in such an environment is a difficult task. FIG. 1A depicts magnetron 100 for illustrative purposes. FIG. 1B shows magnetron 100 integrated into a large glass coating system 130. Glass coating system 130 may be several hundred feet long and contain many magnetrons. Target tube 106 is supported by two end blocks 104 and 108 as glass sheet 109 passes by. The end blocks 104 and 108 generally supply cooling water, support and rotate the target tube, support a stationary magnetic array within the rotating target tube, and transfer the large amounts of electricity needed for the sputtering process. Effectively transferring electrical power to a rotating target tube is also a complex problem. Maintaining electrical isolation in a sputtering process is also crucial to continually laying down a uniform coating on the glass. If the drive system is not properly electrically isolated from the sputtering process, it will affect the quality of coating deposited upon the glass. The sputtered material may in fact also coat the drive and electrical components of the magnetron itself rather than the glass if they are not properly isolated. Aside from resulting in a poor coating, this has many other ramifications on the continuous reliable operation of the magnetron. For further information please refer to xe2x80x9cCoated Glass Application and Marketsxe2x80x9d by Russell J. Hill and Steven J. Nadel, The BOG Group, 1999 (ISBN #0-914289-01-02).
The process of sputter deposition occurs at a high electrical potential, typically in an environment of a vacuum (relative to ambient pressure), with or without the addition of a gas to that environment. This potential is attained in DC operations between distinct anodes and cathodes. Typically the target having the material to be deposited functions as the cathode in DC applications. In the case of AC operations which are achieved by the use of dual rotational targets the targets constantly alternate potential and each provides the other the function of anode and cathode to complete the electrical circuit. For electron transfer between the anode and cathode to occur they must be and remain physically and electrically isolated from each other. The present invention transfers electrical power to and from the rotating target tubes in either DC or AC mode at the high power levels required.
Additionally the materials sputtered are often times conductive of themselves. Highly conductive metals such as silver, gold, copper, nickel, chromium and titanium may be applied. These materials differ depending upon the type (color, reflectivity, etc. . . . ) of film desired. Stray material can and does collect within the operational environment surfaces. If this stray material collection is not managed it can accumulate to an extent that it can lead to the failure of the electrical isolation of the cathode and anode resulting in a short between them or the formation of conductive paths that compromise the electrical isolation of other components within the area. This will lead to poor and uneven film quality and will require that the magnetron be disassembled and cleaned, both extremely undesirable consequences. Downtime of the magnetron, and thus the glass making process, is extremely costly and inconvenient for the glass manufacturer.
In either the case of DC or AC operation there are substantial voltages and currents applied to achieve rapid deposition rates to achieve increased production. This electrical energy needs to be carefully managed to have a controllable process that is efficient and safe. To achieve this several unique and novel features have been designed into the magnetron and its endblocks.
To transfer DC or AC electrical power to or from the target tube there are several particular aspects of each form of electrical power that need to be addressed that are not readily apparent. First, in DC operation current flow is through the cross section of the conductor or interface. Second, in the AC frequency range used for sputter deposition, typically referred to as the mid-frequency range (about 30 to 80 kHz or higher), the current flow occurs along the surface, or skin, of the conductor or interface. Penetration of the current into the conductor is minimal and not an easily modeled theoretical calculation. It is dependent upon the material from which the conductor is formed and upon the frequency of the alternating current. As the frequency increases the penetration into the conductor decreases. Third, in AC operation as current, voltage and frequency increase, a phenomenon known as an inductive heating effect can occur in various electrically conductive materials. The inductive depth and magnitude of the inductive heating varies with the shape, orientation and location of the materials relative to the current path of the AC circuit. The inductive phenomenon in this sputtering application is not well understood and there is little literature or documentation available describing its effects and mitigations for practical application. What is known is that metallic conductors can be inductively heated and that the effect increases in a non-linear manner the closer the secondary material is to the AC circuit path.
Inductive heating only occurs in AC operation. As the AC frequency increases the effect increases for a given voltage and current. Inductive heating occurs when high frequency alternating current travels from one point to another through a conductor. Physical contact with the conductor is not necessary for inductive heating to occur. The alternating current induces alternating electromagnetic flux fields around the conductor. These flux fields induce circular electron flow within electrically conductive materials in the vicinity of the fields. The induced circular electron flows are termed eddy currents. The heating of materials within the alternating flux fields is dependent on physical location, material conductivity, coupling, frequency, and power density. Heating of the material increases as the material comes closer to the conductor, as the material magnetic permeability increases, as the frequency increases, and as the power density increases.
FIG. 2A illustrates a cross section of an ideal target tube. Target tube 110 has opposing faces 110a and 110b. In ideal conditions face 110a and 110b will be parallel to each other and perpendicular to the centerline 110c, the axis of rotation of target tube 110. Ideally, the inner diameter at face 110a will be concentric with the outer diameter at face 110a. Likewise, the inner diameter at face 110b will ideally be concentric with the outer diameter at face 110b, and the inner diameter of the tube will be concentric with the outer diameter anywhere along the length of the tube. In reality, this is rarely true because it is not only difficult to manufacture such a tube, but also to inspect the tube throughout its entire length, and thereafter reject it as out of spec. Furthermore, as discussed earlier, the tube actually changes shape during normal operation as material is sputtered from the tube. Face 110a and 110b may not be parallel in one or often multiple axis of reference, as shown in FIGS. 2B-2D. FIG. 2B illustrates a simple sag of the target tube. FIG. 2C illustrates warpage of the target tube above and below the axis of rotation. FIG. 2C illustrates complex warpage of the target tube wherein the warping occurs in more than one plane. The net result is eccentric rotation of the target tube. The length of target tubes can also vary due to machining variations and also from elongation of the tube as it heats up. This elongation is an additional stress in a rigid support system.
Therefore there must be some allowable tolerance for the variation and imperfection in shape of the target tube. Additionally, improved electrical and thermal isolation is needed to prevent costly downtime of the magnetron and the other machines involved in the manufacturing and coating process.
The endblocks of the cylindrical magnetron provide a unique solution to the problems associated with the operational functions required for sputter deposition of materials utilizing a cylindrical target in DC and AC applications, particularly when high current levels are required for increased rates of deposition.
The present invention adapts to a greater amount of target tube manufacturing and process related variations with angularly compliant mechanisms at each end. The mechanisms also accommodate growth or variations in length of the target tube along the axis. This compliance reduces the transmission of stress within the structure of the magnetron and allows for more consistent and reliable operation.
The invention also simplifies operational alignment, installation, compatibility for retrofit to pre-existing installation sites, assembly, and servicing characteristics.
Electrical isolation is another aspect of the invention. Redundant isolation areas are included to prevent grounding of the device and maintain the floating electrical isolation during operation both initially and over extended periods of usage without maintenance.
Another aspect of the invention are thermal systems to control and minimize the effects of heat generated at static locations where electrical power is provided to the device and at dynamic locations where electrical power is transferred within the device to rotating components. Control and minimization of AC inductive heating is achieved by material selection, construction and geometry taking into account constraints of the sputter deposition process.
Another aspect of the invention incorporates dual water and vacuum sealing to handle the dynamic flow of water through a rotating target in vacuum conditions. The dynamic water seals operate in a primary/secondary set with an unobstructed draining of the interseal area which provides a conditional alarm function while precluding the pressurization of the secondary seal, thereby increasing its operational reliability. The dynamic vacuum seals operate in a primary/secondary set with a differentially pumped interseal area between the primary and secondary seals to provide an effective initial vacuum seal engagement while also providing a backup seal and monitoring between the seals as an added feature for process operations.
Yet another aspect of the invention is the use of chromium oxide surfaces that have been diamond polished to provide wear resistant seal surfaces for both the water and vacuum rotational sealing areas.